U.S. Pat. No. 5,307,369 to Kimberlin, one of the applicants in the instant patent application, discloses in the abstract thereof: “A system for combining a plurality of laser beams into a combined output beam from at least two laser sources includes the removal of conventional perpendicularly oriented output windows from each of the laser sources. Reflecting mirrors are positioned perpendicular to the optical axis at the rear of the two laser sources. A fully reflecting mirror is positioned perpendicular to the optical axis of the first laser source to reflect coherent light received from the first laser source. A beam splitter is positioned between the first laser source and the fully reflecting mirror at the intersection of the optical axes of the first and second laser source. The beam splitter directs a portion of received coherent light into a combined output beam, with the remainder being directed back to the first and second laser sources.”
Referring to FIG. 4 of the instant patent application (a reproduction of FIG. 1 of the '369 Kimberlin patent specification, referred to by reference numeral 400 herein), and col. 4, line 47 to col. 5, line 64, it is stated that:
“In operation, laser output (represented by line 50) is produced by sustained resonant oscillation of coherent light through the laser sources 20 and 22, as controlled by multiple reflections off mirrors 30, 32, 36, and beam splitter 40. These coherent light reflections are represented by lines 51, 52, 55, and 56, which are intended to represent coherent light traveling in both directions along the lines. Laser beam combination proceeds, for example, by simultaneously pumping the active medium of laser sources 20 and 22 with flash lamps (not shown). A photon leaving the laser source 20 can be randomly directed, for example, toward the first reflecting mirror 30. This photon (shown as line 51) is reflected from the mirror 30, and reverses its direction to move back into the laser source 20. Here the photon encounters an active atom at an upper energy, which it stimulates to emit another photon of identical frequency, polarization, and direction. The pair of coherent photons respectively-encounter additional active atoms in the active medium to create still more coherent photons. The coherent photons eventually leave the laser source 20 to pass toward the beam splitter 40. When the coherent photons encounter the beam splitter 40, about 50% are reflected to provide output beam 50. The remainder pass through splitter 40 and proceed (line 54) to be reflected backwards from fully reflective mirror 36 towards the beam splitter 40. Again, about 50% of the coherent photons are reflected, but this time they are directed toward the laser source 22. The remaining coherent photons proceed (line 53) back toward laser source 20. These coherent photons pass through the laser source 20 to create still more coherent photons, which exit the laser amplifier for reflection from the mirror 30. This positive feedback process is multiply repeated to create substantial numbers of coherent photons, at least until the number of active atoms in the active medium drops below substainable (sic, “sustainable”) lasing threshold. A certain percentage of coherent photons directed toward the laser source 22 by reflection from beam splitter 40 also eventually proceed back along line 52 to sustain coherent photon production, similar to that previously described for those coherent photons that travel from line 54 through beam splitter 40 and along lines 53, 52. The coherent photons reflected (line 55) by beam splitter 40 toward laser source 22 pass into the laser source to trigger a coherent photon cascade similar to that described in connection with laser amplifier 20. The coherent photons leave the laser source 22 (line 56), are reflected back into the laser source 22 to trigger production of still more coherent photons. These photons leave (line 55) the laser source directed toward the beam splitter 40. About 50% of the coherent photons pass through the beam splitter 40 are combined with coherent photons arriving from the laser source 20 (lines 52 and 53). The remaining coherent photons are directed (line 54) toward mirror 36, which as previously described reflects the coherent photons toward the beam splitter 40. The process of coherent photon production, with some coherent photons passing through the beam splitter 40 toward laser source 20, and the remaining coherent photons being directed back to laser source 22, is again repeated. Although the exact energy of the combined output beam 50 depends upon the active medium employed, scattering and absorption losses, time and energy of pumping action, and other factors known to those skilled in the art, typically two 400 watt laser sources can be combined as described to produce about 800 watts of laser output with minimal degradation in beam diameter and focus as compared to a 400 watt laser amplifier alone. As will be appreciated by those skilled in the art, pulsed operation is not required for operation of the described embodiment. Low power continuous laser amplifiers can also be combined to double the power of the output beam. In addition, the system 10 as shown in FIG. 1 can also be operated to increase repetition of laser pulses. Instead of simultaneously pumping the laser sources 20 and 22, the laser sources 20 and 22 are alternately pumped, providing a series of laser beams directed along the same optical axis. In this mode, losses due to absorption and scattering in the non-active, “cold” laser source are slightly increased, but the repetition rate of the system can be doubled as compared to a single laser amplifier, while still delivering full rated power.”
The '369 patent to Kimberlin employed a lamp pumping system which produced combined and/or sequenced laser pulses. Two systems sold by Electrox, assignee of the '369 Kimberlin patent, employed a third laser head (laser source) disposed in line 54 of FIG. 1 of the '369 patent to Kimberlin.
A standard laser system 100 illustrated in FIG. 1 comprises a gain medium 101 with a highly reflective rear mirror 103 and a partially reflective output coupler 102 through which the output beam 106 exits. Reference numerals 104, 105 and 106 represent the output axis.
Two current methods for power scaling of laser systems include multiple intra-cavity oscillators as illustrated in FIG. 2 and a MOPA system (Master Oscillator/Power Amplifier) as illustrated in FIG. 3. The multiple intra-cavity oscillator method 200 combines multiple gain mediums 101, 201 between the highly reflective mirror 103 and the partially reflective mirror/50% output coupler 203. Reference numeral 202 represents the optical axes and the output of the second laser head 201. The laser output after the coupler is represented by reference numeral 204.
The MOPA system 300 illustrated in FIG. 3 uses a typically low power oscillator (such as laser head 101) which seeds a second laser 301 via a partially reflective output coupler 302. Reference numeral 303 represents the optical axis and reference numeral 304 represents the laser output.
There are major disadvantages encountered with both of these methods. One of the main disadvantages is that as power is scaled up, some or all of the laser heads have all the laser power being transmitted through them. This causes increased stress and heating on the optical components, substantially lowers the quality of the laser beam, creates lensing and optical waists that shift throughout the system, and severely limits the power scaling achievable. A second major disadvantage is that under both of these methods, the individual lasers cannot lase efficiently independently.
Long pulse width (duration) at high power levels are problematic in that they cause recast layers, heat affected zones, micro-cracking and delamination of materials. Therefore, it is necessary to accurately control the pulse width(s), stacking, sequencing and power of laser beams.